Many vehicles include fluid systems that include a reservoir that contains a reserve volume of fluid to maintain a desired fluid level in the fluid system. The reservoirs typically include an opening or fill spout through which extra fluid may be added, and those openings are closed by a cap. Some fluid systems, for example hydraulic fluid systems such as may be used with power steering systems, are desirably maintained under some pressure. In such systems, the reservoirs and/or cap may be vented or have other valve arrangements to maintain the pressure in the reservoir and fluid system, and/or to avoid an unduly high pressure therein.